The high-frequency peaked BL Lac PG 1553+113 was observed in 2005 with the HESS stereoscopic array of imaging atmospheric-Cherenkov telescopes in Namibia. Using the HESS standard analysis, an excess was measured at the 4.0σ level in these observations (7.6 hours live time). Three alternative, lower-threshold analyses yield >5σ excesses. The observed integral flux above 200 GeV is (4.8±1.3stat±1.0syst)×10-12 cm-2 s-1, and shows no evidence for variability. The measured energy spectrum is characterized by a very soft power law (photon index of Γ=4.0±0.6). Although the redshift of PG 1553+113 is unknown, there are strong indications that it is greater than z=0.25 and possibly larger than z=0.78. The observed spectrum is interpreted in the context of VHE γ-ray absorption by the Extragalactic Background Light, and is used to place an upper limit on the redshift of z<0.74.
Evidence for VHE γ-ray emission from the distant BL Lac PG 1553+113
- F. Aharonian et al. (H.E.S.S. Collaboration)
- Astronomy & Astrophysics 448, L19-L23 (2006)
- 2006
Abstract
Auxiliary informations
H.E.S.S. data points for Figure 2
Energy interval Mean energy Flux Flux Error [TeV] [TeV] [/TeV cm^2 s] 0.185 0.247 0.210146 9.22605e-11 2.94734e-11 0.247 0.329 0.28032 4.04233e-11 9.23071e-12 0.329 0.439 0.373635 6.30641e-12 4.30936e-12 0.439 0.585 0.498305 3.81127e-12 1.7281e-12
Remarks: Statistical errors only. Systematic error on energy scale estimated to 20%. See paper for details.
